How do kidneys filter blood?

Blood enters each nephron through a knot of tiny vessels called the glomerulus, where pressure pushes water and small molecules through into a surrounding capsule. This filtrate then flows through a long tubule, which reabsorbs most of the water, glucose, salts and other useful substances back into the blood. What remains, including urea and excess salts, becomes urine and drains to the bladder. The kidneys filter about 180 litres of fluid a day, yet produce only around 1 to 2 litres of urine. They also regulate blood pressure, balance body salts and make hormones that stimulate red blood cell production.